Q.: I am a ninth grade student and for my science project I have decided to research the subject of concrete. My father has received your magazine for years and I hope you may be of assistance in my project. I would appreciate some information about concrete and an idea for an experiment.
A.: We would like to suggest the subject of cracking due to shrinkage. You may know that after concrete cures it shrinks when it dries. If it is prevented from shrinking freely it may crack. For this reason it is common to put joints in sidewalks, pavements and driveways in the hope that the cracks will form below the joints and not elsewhere.
We would like to refer you to Chapter 12 of a Portland Cement Association book, Design and Control of Concrete Mixtures, Eleventh edition. This chapter discusses the various factors that can cause concrete to have high or low drying shrinkage. Our suggestion for a project would be to study how one or more of these factors affect the amount of cracking due to drying shrinkage. You could make the project as large or small as you want to, depending on how many factors you study and how thoroughly you study each one.
A method of studying the subject would be to make a number of concrete bars about 2x2x12 inches in size with a reinforcing bar running lengthwise through the center of each. Mixes could be made with various water contents, using as wide a range as possible. You should probably make two bars with each water content and then average whatever results you get. After curing, the concrete bars could be allowed to dry, all under the same conditions of relative humidity and temperature. Then you could measure the total number of cracks that develop in each, and, if you could work out a way of doing it, the sum total of crack widths in each concrete bar. This would show the relationship between total water and the cracking due to drying shrinkage. You could include a pair of concrete bars that had no reinforcing bars in them. These would shrink like the others but should not crack because they could shorten freely without being restrained by the steel. You might want to measure how much they shrink (maybe anything up to about 0.006 inch in each 12-inch bar).
You could expand on the project to the extent you wanted by (1) making bars that you would give different amounts of curing, (2) making concretes with various maximum sizes of aggregate up to about 1/2 inch, (3) using various lightweight aggregates, (4) studying the effects of repeated wetting and drying, or (5) anything else that appeals to you.
You could start with a good mix design furnished by someone in your area who knows about such things, and then adjust it for water content or other factors as needed. The reinforcing bar could be Number 4 or larger, but all reinforcing bars should be the same size. In planning the project you would have to devise a mold that could be easily taken apart and reused. The mold would have to be capable of holding the steel bar in the proper position. You would also need a good means of proportioning, mixing, and consolidating your concrete so that all bars are as alike as possible except for the one difference you are studying.